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Graft Selection for ACL Knee Reconstruction

By Melodie Metzger, PhD

Anterior cruciate ligament reconstruction is one of the most common orthopedic surgeries performed worldwide. Despite its frequency, several questions surrounding the procedure continue to spark debate, the most controversial being: "Which graft choice is best?"

The "graft" refers to the new tissue that will be used to reconstruct the patient's torn ligament. The two main options are an autograft, in which new tissue is harvested from the patient's own body, and an allograft, in which the tissue comes from a donor or cadaver.

Each has advantages and disadvantages. For instance, autografts have little to no risk of disease transmission and are more cost-effective, while allografts do not require removal of graft tissue (usually a tendon) from around the patient's knee, reducing surgical time and complications associated with this additional step.

But one potential complication associated with collection of autograft tissue that has not been fully investigated is loss of stability. Hamstring tendons are a popular graft choice that requires removing a patient's own hamstring tendon from the medial portion (inside) of their knee. However, when intact, these medial hamstrings help stabilize the knee by preventing excessive rotation. Other rotational stabilizers, like the medial collateral ligament, are present to help compensate, but an estimated 20 percent of patients injure their medial collateral ligament at the same time they tear their ACL. This presents an interesting clinical question: Should we caution against using hamstring autografts for ACL reconstructions when an MCL injury is present?

To investigate this question, the Orthopaedic Biomechanics Lab at the Cedars-Sinai Orthopaedic Center used a cadaveric knee model. Each knee was subjected to a series of static loads while motion-tracking cameras recorded precise displacements to quantify stability in the knee.

This was repeated for the following clinical scenarios: intact knee, removal of the ACL, removal of the ACL with an MCL injury, and ACL reconstruction with an MCL injury using different graft options, simulated by either applying (i.e., allograft) or removing (i.e., hamstring autograft) a force to the medial hamstrings.

Our results verified that the addition of a partial MCL tear to complete ACL tear significantly decreases rotational stability. In addition, we demonstrated that in the setting of a concurrent MCL tear, ACL reconstruction with the medial hamstrings loaded does not significantly increase rotation compared to intact knees. Conversely, when the hamstrings were unloaded, as would be the case when using a hamstring tendon autograft, there was a significant increase in valgus (knocked knee) rotation (Figure 1).

This research will ultimately help surgeons determine the best graft choice, surgical technique and post-operative bracing for patients with rotationally unstable knees. This work will be presented at the American Orthopaedic Society for Sports Medicine in July and at the Western Orthopaedic Association, where it has been recognized for a Young Investigator Award.

Figure 1. Valgus rotation as a function of knee angle for three of the test conditions, including: Intact with hamstring muscles engaged (Δ), ACL reconstruction with MCL-partial injury with hamstring muscles loaded (◊) and without hamstring muscles loaded (X), which had significantly greater rotation compared to intact across all knee flexion angles (*, p<0.05).